Every year, 250 million people are infected with malaria, resulting in 880,000 deaths, primarily to African children under age five. Many malaria-endemic countries use Indoor Residual Spraying (IRS), the application of dichlorodiphenyl trichloroethane (DDT) or pyrethroid insecticides to interior walls of homes, to control malaria. Concerns regarding the unintended consequences of chronic, high-level DDT exposure have resulted in considerable international debate. For example, the Stockholm Convention on Persistent Organic Pollutants aims to ban DDT, while the World Health Organization and the U.S. Agency for International Development promote and/or fund IRS, including with DDT. DDT and pyrethroids are neurotoxicants in insects and animals. Human studies, primarily conducted in Western populations with low to moderate levels of exposure, suggest that prenatal exposure to DDT and its breakdown product, DDE, may adversely affect children's neurodevelopment. DDT/E exposure has also been related to adverse effects on thyroid function, gestational duration and birth weight, and changes in global DNA methylation, which could serve as pathways for compromised neurodevelopment. Although there is virtually no research in humans on the health effects of pyrethroids, research in rodents has shown detrimental effects on neurodevelopment both independently and synergistically with DDT. No study has evaluated the impact of human exposure to DDT and/or pyrethroids on neurodevelopment in the context of IRS We propose to address these important knowledge gaps by examining relationships between exposure to IRS insecticides and neurodevelopment, and possible mediators in a birth cohort study. We aim to enroll 750 rural South African mothers and to follow their children for 24 months, assessing exposures and neurodevelopment prospectively. We take advantage of a natural experiment where gradations in altitude impact mosquito survival and, by extension, use of IRS, but sprayed and unsprayed villages are otherwise similar, e.g, demographically. Our proposed study will be the first to: 1) assess levels and determinants of IRS insecticide exposure to pregnant women and infants in IRS-treated homes; 2) study the effects of very high DDT/E and pyrethroid exposures on human neurodevelopment; 3) assess the relative and synergistic health impacts of alternative IRS insecticides; and 4) evaluate whether poverty, malnutrition and poor health potentiate the health effects of these exposures. We will introduce two techniques with wider implications for environmental health science: the use of PBPK models to quantify the relative contributions of in utero, breast milk, and other postnatal environmental exposures in infants, and the use of state-of-the-art epigenetic methods to determine whether environmental exposures can alter DNA methylation in fetal tissue. This study aims to provide data to evaluate the risk/benefit of different methods of malaria control and for the development of appropriate interventions to reduce exposure to pregnant women and children.